The invention relates to a steering wheel for motor vehicles and a method for its manufacture.
In conventional steering wheels, the generally metallic skeleton comprises the hub region, the spokes and the steering wheel rim. In the hub region, high material strengths are necessary in order to be able to transfer all the forces occurring in the intended use of the steering wheel in as space-saving a manner as possible onto the steering column which is fastened in the vehicle body. Also the spoke and rim regions of the steering wheel skeleton must be designed primarily for the steering forces which are to be transferred. Furthermore, these regions of the steering wheel skeleton have already been designed for some time such that in the case of an impact of the driver of the vehicle caused by an accident, they can absorb a high proportion of the impact energy by deformation and hence reduce the risk of injury.
Steering wheel skeletons are either constructed from metallic semifinished parts which are preformed in a machining or non-machining manner and which are connected with each other by means of welding or soldering processes, or by a casting technique and then are generally produced in one piece from a light metal alloy. In so doing, of course one strives to meet all mechanical requirements with as small an expenditure of material as possible, the manufacturing methods by casting technique being in fact generally more expensive, but offering substantially more possibilities in fashioning compared with the manufacture from semifinished shaped parts.
A further essential aspect in the design of steering wheel skeletons results from the necessity of fastening the airbag module, which in all modern motor vehicles is arranged as standard in the steering wheel. With the explosion-like unfolding of the airbag, quite considerable reaction forces arise for a short period of time, which have to be transferred reliably from the airbag mount to the steering wheel skeleton.
By the invention a steering wheel for motor vehicles is made available which fully or almost completely does away with the steering wheel skeleton that would increase the manufacturing expense.
The steering wheel according to the invention has a hub, a steering wheel rim and a plurality of spokes connecting the steering wheel rim with the hub. The hub comprises a hub core for fastening to a steering shaft and a base body which surrounds the hub core and is made of a fiber-reinforced plastic material. The spokes are integrally and continuously molded with the base body of the hub from a fiber-reinforced plastic material. The steering wheel rim is integrally and continuously molded with the spokes from a fiber-reinforced plastic material. The base body together with the spokes and the steering wheel rim forms a self-supporting shaped body. Suitable plastic materials are the same as have been used hitherto for encasing the steering wheel skeletons. These plastic materials are in particular polyurethane, polyamide, and polypropylene. Preferably, these plastic materials are also foamed, just as in the casings usual hitherto. It has been found that by adding reinforcing fibers to the plastic mass there can be achieved such a mechanical strength of the steering wheel body that the use of a steering wheel skeleton is no longer necessary. It is possibly in the region of the hub of the steering wheel that the use of a reinforcing means is expedient which at the same time serves then for supporting and mounting a gas bag module.
In the preferred embodiment there is achieved a particularly high strength and solidity of the steering wheel in that the shaped body in the region near its outer surface is reinforced by fibers which are arranged predominantly stretched and parallel to the outer surface; the shaped body can be reinforced internally by irregularly or randomly distributed fibers, the fibers, however, should be present predominantly in a stretched condition.
The invention further provides a method for the manufacture of the steering wheel. The exceptional feature of the method is that the hub core of the steering wheel is placed into a mold and a mass, consisting of plastic material and reinforcing fibers, is introduced into the mold. The mass is introduced without pressure into an open form tool part preferably by means of a program-controlled and movable injection head. In the process, the reinforcing fibers are added to the injected mass in the course of injection molding. When injection of the mass is completed and prior to foaming up the latter, the mold is closed. The metering of the reinforcing fibers is preferably done in the course of injecting the mass, corresponding to the desired mechanical properties of the various regions of the steering wheel, and likewise preferably program-controlled.
By employing the method according to the invention it is achieved in a particularly advantageous manner that the reinforcing fibers are arranged in the region near the outer surface predominantly stretched and parallel to the outer surface, whilst in the interior of the shaped body they may also be arranged irregularly and randomly. The reinforcing fibers, however, are predominantly present in a stretched condition. The reinforcing fibers constitute a shell of the shaped body, which allows to be loaded with high tensile and compressive forces. Due to this nature of the shaped body, it has the desired mechanical properties, in particular an appropriate flexural strength and the plastic deformability which is necessary for energy dissipation in the case of an impact.